1. Field
The present disclosure generally relates to alignment structures for integrated-circuit packaging. More specifically, the present disclosure relates to alignment structures for use in a multi-chip module (MCM) and techniques for fabricating the MCM.
2. Related Art
Multi-chip modules (MCMs) are being developed to facilitate proximity communication (such as capacitively coupled communication) between multiple integrated circuits (ICs) for the next generation of high-performance computers. In these MCMs, adjacent chips are often positioned face-to-face so that information can be communicated between proximity connectors (such as metal pads) on the surfaces of the facing chips.
In order to enable ultrafast chip-to-chip communication speeds via capacitive coupling between the metal pads on the facing chips, these chips need to be aligned with each other on a micrometer scale. In some proposed MCMs, the alignment is obtained by using micro-spheres or balls that mate with pairs of opposing etch pits on the surfaces of facing silicon chips. However, this ball-and-pit alignment technique can be expensive because of: the cost of silicon substrates; and the active regions on the silicon substrates that are occupied by the etch pits. For example, the gap between adjacent chips for proximity communication is typically around 10 μm, while the ball-and-pit alignment technique may consume around 200×200 μm2 of a silicon substrate when 200-μm diameter balls are used.
In addition, it can be difficult and expensive to assemble the MCMs because of: the difficulty of positioning the balls in the etch pits (which becomes more difficult as the size of the balls is reduced); the fact that the three components (two silicon substrates with etch pits and at least one ball) have additional degrees of freedom, which makes assembly more complicated; and the fact that the ball-and-pit alignment technique typically is not compatible with other, lower-cost substrates (such as organic or ceramic substrates). As a consequence, the costs of MCMs that use ball-and-pit alignment can be prohibitive.
Hence, what is needed is an MCM and an associated fabrication technique which do not suffer from the above-described problems.